The term is has two primary usages:
1) can be used as a term to express life, personal to you as an individual.

2) is widely used as a term in East Asian countries as a way to addressed your beloved one. The person you love becomes your everything your life, therefore the ancient sages and habitants of EA began calling their partners/lovers/spouses Jaan as a way to express their deep affection for them. Usually, your Jaan would have the following characteristics:
- they complete you
- they are always in your mind, thoughts, heart
- they care deeply for you
- they love you
- they are kind, genuine, sweet, caring, understanding, honest, loyal, faithful
- no matter how wonderful and unwonderful times are, they always stand by you
- they have big hearts
- they make your heart skip beats
- you would do anything for them
- they are the reason you smile day in day out
- you can see yourself spending the rest of your life with them
- they gift you with goodies such as hugs, kisses and comfort
- you can see in their eyes a sincerity
- they are your best buddy
- you can share with them anything and they are trusted
- you love them with all your heart
- you can't imagine life with out them
- they are your life

*(Include but are not limited to the above)

look at those two over there eating carrot sticks... she is definitely his Jaan